1. Field of the Invention
The present invention relates to an improvement in the methods means for controlling an aperture of a camera, wherein the aperture stop-down position is controlled by the number of applied pulses.
2. Description of the Prior Art
Generally small-sized cameras have so far adopted an exposure control systems such as a programmed system wherein a shutter speed and an aperture value are determined, and an exposure is controlled by film speed (e.g., ASA) of a photofilm loaded in a camera and the photometric result of the luminance of a photosubject; a shutter-speed priority aperture system wherein an aperture value is determined and controlled by film speed (e.g., ASA) of a photofilm, a photometric result and also a pre-set shutter speed; and a system wherein an aperture value is manually determined and controlled taking priority of a shutter speed. Such methods for controlling aperture blades have been those that an aperture stop-down detector is attached to an aperture control lever motivating aperture blades to operate, and either counted value obtained by the use of an aperture stop-down detector comprising a combination of a pulse generator and a counter, or a resistance value obtained by the use of an aperture stop-down detector comprising a resistor is compared with either preset value of counts or preset value of resistance correspondingly; the aperture control lever is stopped by a stopper mechanism at the position where the both values compared become equivalent to each other, and thus a stop-down operation can be controlled. In this method, a time lag between a detection of an aperture stop-down value obtainable in the form of a counted value or a resistance value and a stop of an aperture control lever has occurred, and this has been the cause of deterioration in aperture accuracy. Accordingly, one cannot decrease the aperture closing-down speed of aperture blades to secure an accurate aperture control.
As for the other methods for controlling aperture blade operation to secure the accuracy thereof, there are known, for example, a feed-back control method wherein a DC motor is used and a method using a pulse motor. In case of the former, the control circuits and mechanism become complicated. On the other hand, in case of the latter, the mechanism thereof becomes simpler and more accurate, but the number of pulses per hour to which a pulse motor responds is limited; therefore it cannot be used for a rapid aperture control.
The present invention relates to an improvement of aperture control using a pulse motor or the like. A description will be made at first particularly with reference to the aperture control, wherein a pulse motor is used. Nowadays, the self-electromotive frequency of normal type of pulse motor is up to the order of 1,000 pps (pulses per second). For example, if F1 is fixed as the fundamental and an aperture control at F22+0.5 EV is done for every 0.1 EV as a unit, the aperture steps between F1 and F22 are nine steps of F1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22, i.e., for 9 EV that would be 90 pulses, and for 0.5 EV it would be 5 pulses, therefore the total pulse numbers would be 95. Now, if an aperture control of F22+0.5 EV, wherein F1 is fixed as the fundamental, is tried within the time of 50 ms (0.05 sec.), the equation, 95.div.0.05=1,900 pps would be formulated, therefore it cannot be feasible with the use of a normal type pulse motor. This fact means that any normal type pulse motor cannot control an aperture operation by every 0.1 EV as a unit.